The present invention is related to digital communication systems and more particularly to systems and methods for controlling output power of subscriber units in a point to multipoint communication system.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
In order to conserve scarce spectrum, the data communication devices of a point to multipoint wireless communication system may share access to a common frequency. In a typical scenario one or more frequency channels are allocated to downstream broadcast communication from a central access point to a plurality of subscriber units and one or more separate frequency channels are allocated to upstream communication from the subscriber units to the central access point. For upstream communication there is a medium access control (MAC) protocol that determines which subscriber unit is permitted to transmit at which time so as not to interfere with transmissions from other subscriber units.
For a given upstream frequency, the time domain is divided into frames which are typically of equal duration. Each frame represents an individually allocable unit in the time domain. One subscriber unit transmits in each frame. Reservations for transmission in a particular frame are made by the central access point and distributed in broadcast downstream transmissions. Such a scheme is referred to as a time domain multiple access scheme (TDMA).
In such a point to multipoint wireless communication system, it is generally preferable to centrally control the transmission power of each subscriber unit. Each subscriber unit should transmit at a power sufficient to ensure accurate reception its transmission yet not so high so as to overload the front end of the central access points receiver or cause interference to unintended receivers. Power control involves monitoring received power for each subscriber unit at the central access point and sending power adjustment information downstream to maintain power at the desired level.
Cable modem systems also involve access to a shared medium and also require subscriber unit power control. It would be desirable to simply adopt a MAC protocol already developed for cable applications to the wireless context. One such protocol that has been developed is referred to as MCNS protocol. The MCNS protocol is described in the Data-over-Cable Service Interface Specifications, Radio Frequency Interface Specification, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents of which are herein incorporated by reference.
A cable MAC layer like MCNS is already implemented in low cost chip sets. The operational characteristics of MCNS are well known. Furthermore, it is desirable to maintain parts commonality between wireless modems and cable modems to the extent possible.
The MCNS protocol provides for controlling the power of subscriber units. In one implementation, the power control function is combined with monitoring of the round trip propagation delay between the central access point and individual subscriber units. Periodically, the central access point sends a ranging request message to a particular subscriber unit. In response to the ranging request message, the subscriber unit sends a ranging response to the central access point. Based on this transmission, the central access point establishes a round trip propagation delay and sends this value to the subscriber unit. The central access point measures the power level of the ranging response message. Based on the power measurement, the central access point sends the subscriber unit power adjustment information to help the subscriber unit set its power so that it will be received at a desired level.
This combined ranging and power control operation is, however, relatively infrequent, occurring approximately every two seconds in a typical implementation. This MAC layer power control operation cannot easily be made more frequent because of the limited processing power provided by equipment implementing the MCNS protocol. Also, each subscriber unit's ranging response requires a MAC frame, causing frequent updates to reduce system efficiency.
In a wireless system, the frequency of power control feasible with the use of MCNS power control features is insufficient due to the inherently more rapid variation in wireless channel response over time. What is needed are systems and methods for providing more rapid update of subscriber unit power level while still interoperating with wireline MAC protocols.